198 



V. Alexander et al. 



I - 



E 

 u 



a. 

 «> 

 o 



S 3 



E 



» 



Chlorophyll a., /ig (g sediment) 



100 200 



— I — I — I — I — I — I — r 



1 — r 



Chlorophyll a 



Photosynthesis 



J L 



_L 



_L 



J_ 



100 



Potential Photosynthesis 

 ^g C(g sediment) hr 



200 



J L 



FIGURE 5-11. Potential photosyn- 

 thesis and chlorophyll a at different 

 depths in the sediment of Pond B, July 

 1972. 



The data obtained in this way are estimates of potential production 

 (Grontved 1962) rather than of true production per square meter. These 

 values were converted to true production rates by the method suggested by 

 Hunding (1971) which uses measurements of photosynthesis as a function 

 of light intensities. These determinations, plus measures of the actual light 

 extinction within the sediment, allow photosynthesis rates per square 

 meter to be calculated. 



Most of the primary production of the epipelic algae occurred in the 

 top millimeter of sediment (Figure 5-12) because of the rapid extinction of 

 light. The algae are abundant in the top 4 cm or so and are capable of 

 photosynthesis when incubated in the light (Figure 5-1 1). 



The seasonal curve of photosynthesis in Pond J in 1971 (Figure 5-10) 

 is typical of the results. Rates of photosynthesis began to increase soon 

 after the ponds thawed and warmed; the rates rose from 2 mg C m "^ hr " ' 

 on 21 June to around 16 mg C m "" hr"' by mid-July. Following this peak, 

 there was a steady decline throughout late July and August. Presumably 

 photosynthesis continued until the ponds froze in mid-September. These 



